Blogging tools have several characteristics that make them a CMS. And they are very important, because arguably as much content is being managed in weblogs as in all the content in more standard CM systems.

Weblogs have a very simple information architecture. Their main organizing principle is date and time (reverse chronological ordering of blog posts). More sophisticated blog tools add arbitrary categories, as do all news portals. Indeed, early news portals like Slash were often called weblogs.

Blogging tools separate content from presentation (layout). This is a major time saver when the blog needs to be redesigned. Change the template, and hundreds of pages change together.

They often separate the development staging server from the delivery server.

They use templates and feed content objects into the templates. Templates can be edited, but tools do not offer a WYSIWYG template editor.

They use RSS aggregation of feeds and syndication of stories. The News Reader or Aggregator allows weblog subscribers to scan quickly all the latest posts from many blogs.

They simplify hyperlink references to posts in other blogs. See Trackback and Pingback.

They have WYSIWYG editors.

They can have multiple authors (team blogs), but rarely have any workflow.

There has been some effort toward a BlogML to facilitate semantic markup with XML, but that has largely failed.

There is a Blogger API that allows independent client tools to post to various blogs.

As with CMS, you can buy blogging software and run it on your own web server (notably Manila from Userland and Movable Type), or build with open-source blogging tools (Blosxom, Greymatter, LiveJournal), but most are provided as ASP hosting services. The great majority of individuals blogging are doing it on "free" hosts like Blogger's Blogspot.com, now owned by Google.